Board-level conformal EMI shield having an electrically-conductive polymer coating over a thermally-conductive dielectric coating

Abstract

An electrically continuous conformal EMI protective shield for adhering directly to and conforming with surfaces of a printed circuit board is disclosed. The conformal EMI shield includes a thermally conductive dielectric coating adhering directly to surfaces of the printed circuit board to provide an electrically nonconductive, thermally conductive, contiguous coating that covers all such printed circuit board surfaces. The conformal EMI shield also includes a conductive coating adhering directly to surfaces of the dielectric coating to provide an electrically conductive layer that prevents electromagnetic emissions from passing through the conformal EMI protective shield.

1. A printed circuit board comprising:
 a printed wiring board; 
 at least one component mounted on the printed wiring board; 
 a thermally conductive dielectric coating adhered to surfaces of the printed circuit board and comprising an electrically nonconductive thermal loading material, wherein the dielectric coating is formed from a dispersion with a viscosity and adhesion sufficient to enable the dispersion to be applied via spray techniques, wherein said dispersion viscosity and adhesion prevents dewetting when said dispersion is applied to surfaces of the printed circuit board, wherein the thermal loading material is 10%–80% and the binder is 90%–20% by weight of the dispersion; and 
 an electrically conductive coating adhered to surfaces of the dielectric coating. 
 
   
   
     2. The printed circuit board of  claim 1 , wherein the dispersion comprises: a base liquid; a binder material suspended in the base liquid; and the electrically nonconductive thermal loading material suspended in the base liquid. 
   
   
     3. The printed circuit board of  claim 2 , wherein the binder material comprises one of the group consisting of acrylic and urethane. 
   
   
     4. The printed circuit board of  claim 2 , wherein the base liquid and binder material are provided in an intermediate dispersion subsequently doped with the thermal loading material. 
   
   
     5. The printed circuit board of  claim 2 , wherein the base liquid is one of either water or an organic solvent. 
   
   
     6. The printed circuit board of  claim 5 , wherein the organic solvent comprises one or more of the group consisting of:
 N-Methyl-Pyrolidinone (NMP); 
 Methyl-Ethyl-Ketone (MEK); 
 an acetone; and 
 an alcohol. 
 
   
   
     7. The printed circuit board of  claim 2 , wherein the thermal loading material comprises a 0.1–10 micron boron nitride powder. 
   
   
     8. The printed circuit board of  claim 2 , wherein said suspension of said thermal loading material is substantially uniform when said dielectric dispersion is applied to said printed circuit board surfaces. 
   
   
     9. The printed circuit board of  claim 2 , wherein said dispersion further comprises one or more of the group consisting of:
 at least one photosensitizing agent to enable said dispersion to be UV cured; and 
 at least one heat-curing agent to enable said dispersion to be temperature cured. 
 
   
   
     10. The printed circuit board of  claim 9 , wherein said photosensitizing agent is a UV-curable acrylic. 
   
   
     11. The printed circuit board of  claim 9 , wherein said heat-curing agent is an anhydride. 
   
   
     12. The printed circuit board of  claim 1 , wherein the thermal loading material comprises one of the group consisting of boron nitride (BN), aluminum oxide (AlO 3 ) and magnesium oxide (MgO). 
   
   
     13. The printed circuit board of  claim 12 , wherein said boron nitride comprises one or more of the group consisting of:
 hexagonal boron nitride; and 
 diamond boron nitride. 
 
   
   
     14. The printed circuit board of  claim 1 , wherein the thermal loading material has a conductivity of at least 20 W/mK. 
   
   
     15. The printed circuit board of  claim 1 , wherein the thermal loading material has a conductivity of at least 100 W/mK. 
   
   
     16. A printed circuit board comprising:
 a printed wiring board; 
 at least one component mounted on the printed wiring board; and 
 a thermally conductive dielectric coating adhered to surfaces of the printed circuit board, wherein the dielectric coating is formed from a dispersion comprising a base liquid, a binder material suspended in the base liquid, and an electrically nonconductive thermal loading material suspended in the base liquid. 
 
   
   
     17. The printed circuit board of  claim 16 , wherein the printed circuit board further comprises:
 an electrically conductive coating adhered to surfaces of the dielectric coating. 
 
   
   
     18. The printed circuit board of  claim 16 , wherein the thermal loading material comprises boron nitride (BN). 
   
   
     19. The printed circuit board of  claim 16 , wherein the thermal loading material comprises aluminum oxide (A103). 
   
   
     20. The printed circuit board of  claim 16 , wherein the thermal loading material comprises magnesium oxide (MgO). 
   
   
     21. The printed circuit board of  claim 16 , wherein the binder material comprises one of a group consisting of acrylic and urethane. 
   
   
     22. The printed circuit board of  claim 16 , wherein the base liquid is one of either water or an organic solvent. 
   
   
     23. The printed circuit board of  claim 22 , wherein the organic solvent comprises one or more of the group consisting of:
 N-Methyl-Pyrolidinone (NMP); 
 Methyl-Ethyl-Ketone (MEK); 
 an acetone; and 
 an alcohol. 
 
   
   
     24. The printed circuit board of  claim 16 , wherein the dispersion has a viscosity and adhesion that prevents dewetting when the dispersion is applied to surfaces of the printed circuit board. 
   
   
     25. The printed circuit board of  claim 16 , wherein the thermal loading material has a conductivity of at least approximately 20 W/mK. 
   
   
     26. A printed circuit board comprising:
 a printed wiring board; 
 at least one component mounted on the printed wiring board; 
 a thermally conductive dielectric coating adhered to surfaces of the printed circuit board and comprising an electrically nonconductive thermal loading material having a thermal conductivity of greater than or equal to 36 W/mK, and wherein the thermal loading material comprises boron nitride (BN); and 
 an electrically conductive coating adhered to surfaces of the dielectric coating. 
 
   
   
     27. The printed circuit board of  claim 26 , wherein the thermally conductive dielectric coating is formed from a dispersion comprising a base liquid; a binder material suspended in the base liquid; and the thermal loading material suspended in the base liquid. 
   
   
     28. The printed circuit board of  claim 27 , wherein the binder material comprises one of the group consisting of acrylic and urethane. 
   
   
     29. The printed circuit board of  claim 27 , wherein the base liquid is one of either water or an organic solvent. 
   
   
     30. The printed circuit board of  claim 27 , wherein the dispersion has a viscosity and adhesion that prevents dewetting when the dispersion is applied to surfaces of the printed circuit board. 
   
   
     31. A printed circuit board comprising:
 a printed wiring board; 
 at least one component mounted on the printed wiring board; and 
 a thermally conductive dielectric coating adhered to surfaces of the printed circuit board and comprising an electrically nonconductive thermal loading material having a thermal conductivity of at least approximately 20 W/mK; and 
 wherein the thermally conductive dielectric coating is formed from a dispersion comprising a base liquid; a binder material suspended in the base liquid; and the thermal loading material suspended in the base liquid. 
 
   
   
     32. The printed circuit board of  claim 31 , wherein the binder material comprises one of the group consisting of acrylic and urethane. 
   
   
     33. The printed circuit board of  claim 31 , wherein the base liquid is one of either water or an organic solvent. 
   
   
     34. The printed circuit board of  claim 31 , wherein the dispersion has a viscosity and adhesion that prevents dewetting when the dispersion is applied to surfaces of the printed circuit board. 
   
   
     35. A printed circuit board comprising:
 a printed wiring board; 
 at least one component mounted on the printed wiring board; 
 a thermally conductive dielectric coating adhered to surfaces of the printed circuit board and comprising an electrically nonconductive thermal loading material having a thermal conductivity of greater than or equal to 36 W/mK, and wherein the thermal loading material comprises magnesium oxide (MgO); and 
 an electrically conductive coating adhered to surfaces of the dielectric coating. 
 
   
   
     36. The printed circuit board of  claim 35 , wherein the thermally conductive dielectric coating is formed from a dispersion comprising a base liquid; a binder material suspended in the base liquid; and the thermal loading material suspended in the base liquid.